PROCESS OF PREPARATION OF CORROSION INHIBITED
COMPLETION FLUIDS FOR OIL AND GAS WELLS
Field of the invention
5
[0001] The present invention relates to a process of corrosion inhibition of oil and gas
wells. In particular, the present invention provides a process for mitigating corrosion
of carbon steel tubing and casing of oil and gas wells in presence of corrosion
inhibiting co~npletionf luids at high temperature and 1iigI:h pressure.
10
Background of the invention
[0002] Oil and gas are produced from underground reservoirs in the Earth. A well is
bored by drilling a hole up to the target depth and stabilized by placing steel tubes and
15 casing in a telescopic design. Further, drilling is carried out using drilling fluid to
replace rock cuttings and stabilizing the drilled hole. Once the target depth is attained,
all the drilling fluid is removed and replaced with well completion fluid. The well
completion fluids are prepared based on the requirements of the colnpletion such as
temperature, pressure and surrounding fol-mations. Further, the well completion fluid
20 is left in the annular region of the oil and gas well between tubing and casing above a
packer. The well tubing and casing made up of carbon steel are in constant contact
with the well completion fluid and are therefore susceptible to corrosion which may
damage the well and lead to loss of oil and gas. Also, damage to casing leads to
contamination of water reservoir and poses a hazard due to gas leak at well head.
25 Therefore, it is important to ensure that well completion fluids meet all the
requirements to fulfil tlicir primary functions such as providing pressure control,
preventing formation fluid from entering the well bore, maintaining hole stability,
minimizing damage of production zone, minimizing corrosion of down hole metals
and providing carrying capacity for debris, cuttings and loose sand. Further, the well
5 completion fluids should be free of solids, sufficiently dense to control the producing
reservoir pressure, resistant to viscosity changes over long periods of time and
noncorrosive to the well bore and completion components in order to perform the
abovementioned functions effectively.
10 [0003] Conventionally, well completion fluids are prepared by dissolving one or more
salts such as zinc bromide and calcium bromide in water based on the underground
reservoir pressure. However, the abovementioned well completion fluids suffer from
various disadvantages. For example, single-salt brines have density limitations.
Further, brines containing zinc bromide salt have the highest corrosion rates because
15 pH of these brines is low. The above limitation is overcome by using combination of
two and more salts to provide adequate density in order to adhere to l~ydrostatic
pressure requirements. However, brines made using combination of salts also suffer
from various limitations. The acidic environment is manifested by brines made up of
combination of two or more salts as well. Also, during the process of preparation,
20 oxygen from the atmosphere gets dissolved in the brines which increases corrosivity.
Corrosivity further increases with the increase in bottom hole temperature, bromide
content and salt content.
[0004] In light of the above-mentioned disadvantages, there is a need for a process of
25 preparation of corrosion inhibiting completion fluids for oil and gas wells. Further,
thcrc is a nced for a proccss that climinatcs dissolved oxygcn from thc wcll
completion fluids during preparation thereby making the well completion fluid less
corrosive. Furthermore, there is a need for a process which facilitates buffering high
density well con~pletionf luids which are highly acidic in nature. In addition, there is a
5 need for well completion fluids that are sufficiently dense and have adequate specific
gravity.
Sunlmarv of the invention
10 [0005] A process for preparing a corrosion inhibited completion fluid for oil and gas
wells is provided. The process comprises a step of adding a pH moderating agent to
water. The process further comprises a step of adding an oxygen scavenger to the
solution containing water and the pH moderating agent. Furthermore, the process
comprises a step of blending one or more salts to the solution after the step of adding
15 the oxygen scavenger, wherein the one or more salts comprise at least one of zinc
bromide and calcium bromide. In addition, the process comprises a step of adding an
acid corrosion inhibitor after the step of blending the one or more salts.
[0006] In an embodiment of the present invention, the pH moderating agent is sodium
20 bicarbonate in an amount of 1.5%. In an embodiment of the present invention, the
oxygen scavenger is sodium sulphite in an amount of 63 milligrams per litre. In an
embodiment of the present invention, quantity of the one or more salts blended with
the solution is based on required specific gravity of the completion fluid. In an
embodiment of the present invention, specific gravity of the prepared corrosion
25 inhibited completion fluid is between 1.20 to 2.00. In an embodiment of the present
invention, 3% acid corrosion inhibitor is added after thc step of blending the one or
more salts.
[0007] A corrosion inhibited completion fluid is provided. The completion fluid
5 comprises water as a base fluid. The colnpletion fluid further comprises a pH
moderating agent. Furtherinore, the completion f-luid comprises an oxygen scavenger.
In addition, the completion fluid comprises one or more salts, wherein the one or
more salts comprise at least one of zinc bromide and calcium bromide. Also, the
completion fluid colnprises an acid corrosion inhibitor.
10
Detailed description of the invention
[0008] A process of preparation of corrosion inhibiting completion fluids for oil and
gas wells is described herein. The invention provides for a process that eliminates
15 dissolved oxygen from the well completion fluids during preparation thereby making
the well colnpletion fluid less corrosive. Further, the invention provides for a process
for moderating the pH of the well completion fluids thereby making the well
completion fluids less acidic. Furthennore, the invention provides for a process for
preparing well completion fluids having wide range of specific gravities.
20
[0009] The following disclosure is provided in order to enable a person having
ordinary skill in the art to practice the invention. Exemplary embodiments are
provided only for illustrative purposes and various modifications will be readily
apparent to persons skilled in the art. The general principles defined herein may be
25 applied to other embodiments and applications without departing from the spirit and
scope of the invention. Also, the terminology and phraseology used is for the purpose
of describing exemplary embodiments and should not be considered limiting. Tlius,
the present invention is to be accorded the widest scope enco~npassing numerous
alternatives, modifications and equivalents consistent with the principles and features
5 disclosed. For purpose of clarity, details relating to technical material that is known in
the technical fields related to the invention have not been described in detail so as not
to unnecessarily obscure the present invention.
[OOIO] The invention provides for a process of preparation and cornposition of well
10 completion fluids for oil and gas wells.
LO01 I] The well conipletioti fluids of the present invention use technical water
(hereinafier referred to as water) as a base fluid. Sodium bicarbonate is added to the
water. Sodium bicarbonate is used as a pH moderating agent and raises the pH of the
15 solution. In an embodiment of the present invention, 1.5% sodium bicarbonate is
added to water. In an embodiment of the present invention, any other suitable pH
moderating agent may be used to raise the pH of the solution. On dissolution of
sodium bicarbonate in water, an oxygen scavenger is added to the solution. In an
embodiment of the present invention, sodium sulphite is added to the above solution
20 to eliminate dissolved oxygen in the solution. In an exetnplary embodiment of the
present invention, dosing of 63 ppm of sodium sulphite is added to the above solution
to remove dissolved oxygen.
[0012] The above solution is then blended with a salt such as, but not limited to,
25 calcium bromide, zinc bromide and combination of both calcium bromide and zinc
6
bromide. In an embodi~nent of the present invention, the quantity of the salt added to
the above solution is based on the requirement of specific gravity to be maintained for
a particular oil and gas well. Further, well completion fluids having wide ranges of
densities and specific gravity are obtained by varying and monitoring the amount of
5 salt added to the solution. In an embodiment of the present invention, the specific
gravity of the solution after dissolving the salt may range from 1.20 - 2.00 depending
on the quantity and types of the salt dissolved.
[0013] Once the salt is added, an Acid Corrosion Inhibitor (ACI) is added to the
10 above solution. The ACI is a free flowing and homogenous organic liquid, at 24 *
2"C, which is free from visible impurities. Further, the qualitative test for the presence
of Arsenic is negative for the ACI. Furthermore, the pour point of the ACI is not more
than 12°C. In an embodiment of the present invention, 3% dosing of the ACI is added
to the above solution. In an embodiment of the present invention, any ACl
15 fonnulation adhering to the above-mentioned properties is used.
[0014] The corrosion rates of steel in presence of the well completion fluids obtained
using the above-mentioned process were calculated based on test parameters
simulating the high temperature and high pressure oil and gas wells. The test
20 parameters are mentioned below:
Test Temperature: 150 * 2°C
Test Pressure: 1000 psi
Condition of Test: Static
Duration of Test: 6 hours
25 Acid VolumeITest Coupon Surface Area: 75 ml Isq. inch
7
Type of Steel: N - 80
Type of Acid: Hydrochloric (1 5% w/w)
Concentration of ACT: 30 ml/l or 3%
Corrosion without pitting: 0.024g/cm2 (Maximum)
5
[OOI 51 During testing, coupons of N-80 steel were fabricated using API 5 CT N-80
steel casing pipe material. The coupons were wet ground to a surface finish of 400
grit. The coupons were then degreased with xylene, washed with distilled water and
10 rinsed with acetone and dried in dry hot air. The surface area and the initial weight of
the coupons was tllen detennined. Subsequently, the coupons were immersed in
PARR 4571 HPHT Autoclave filled with the well completion fluids prepared using
the above~nentioned process. The immersed coupons were exposed to the simulated
test conditions. After the completion of exposure tests, retrieved coupons were
15 washed with water, rubbed against filter paper, rinsed in Clark's solution, followed by
thorough washing in water with 1% non-ionic detergent. Finally, the coupons were
washed in distilled water and rinsed in acetone and dried in dry hot air. The coupons
were then weighed to determine the weight loss in the coupons and the corrosion rate
in millimetre per year and lnilli inch per year.
20
[0016] Based on the weight loss in the coupons, corrosion rate in milli inch per year
(mpy) was calculated using the following formula:
Corrosion Rate = (3.45 X 1 o6 x W) / (A x T x D)
25 wherein,
W: Coupon Weight Loss in gams
A: Surface area in centiinetre2
T: Exposure Time hours
D: Density in grams/centimeter3
5 Corrosion rate in millimetre per year = 0.0254 x Corrosion rate in nlilli inch per year
[0017] The experinlents were repeated with brines prepared using the referenced
invention and effectiveness of corrosion inhibition process was determined by
calculating percentage inhibition as per the fornlula given below:
10 Inhibitor Efficiency in % = (A - B) x 100 / A
wherein,
A = Corrosion rate for blank
B = Corrosion rate with Inhibitor
15 [0018] The results of the test are provided in the tables below:
Table 1 : Results of corrosion rate studies for API 5CT N-80 steel coupon exposed to
Calcium Bromide brine (specific gravity 1.40) at 1 50°C
S. Medium Corrosion Rate
No mPY (lnm/~)
1 Calcium Bromide brine of Specific 22.56
gravity 1.40 (0.57)
2 Calcium Bromide brine of Specific 8.3 1
gravity 1.40 + 3% ACI (vlv) (0.2 1)
3 Calcium Bromide brine of Specific 5.99
gravity 1.40 +1.5% NaHC03 + (0.15)
63ppm Sodium sulphite + 3%
ACI(v/v)
Inhibition
%
0.00
63.16
73.45
Table 2: Results of corrosion rate studies for API 5CT N-80 steel coupon exposed to
Calciuln Bromide brine (specific gravity 1.85) at 1 50°C
5
S. Medium Corrosion Rate Inhibition
No mPY (mm/y) %
1 Calcium Bromide brine of specific 17.81 0.00
gravity 1.85 (0.45 )
2 Calcium Bromide brine of specific 15.73 1 1.67
gravity 1.85 + 3% ACI (vlv) (0.39)
3 Calciunl Bromide brine of specific 9.20 48.34
gravity 1.85 + 1.5% NaHC03 + (0.23 )
3% ACI (vlv)
4 Calcium Bromide brine of specific 4.40 75.29
gravity 1.85 + 1.5% NaHC03 + (0.1 1 )
63ppm Sodiuln sulphite + 3% ACI
(v/v) - - --~-- ---
Table 3: Results of corrosion rate studies for API 5 CT N-80 steel coupons exposed
to Zinc Bromide brine (specific gravity 1.20) at 1 50°C
S. Medium Corrosion Rate Inhibition
No lnPY ( l n d ~ ) '30
1 Zinc Bromide brine of specific gravity 203.30 0.00
1.20 (5.08 )
2 Zinc Bromide brine of specific gravity 7.91 96.10
1.20) + 1.5% NaHC03 + 63 ppm (0.20 )
Sodiuln sulphite + 3% ACI (v/v)
Table 4: Results of corrosion rate studies for API 5 CT N-80 steel coupons exposed
to Zinc Bromide brine (specific gravity 1.85) at 1 50°C
IS. I Medium I Corrosion Rate 1 Inhibition 1
10
N o lnPY (mm/~) %
1 Zinc Bromide brine of specific gravity 8680.68 0.00
1.85 (2 17.02)
2 Zinc Bromide brine of specific gravity 49.86 99.43
1 I 1.85 +1.5 % NaHC03 + 3% (1.25 )
1.85 + 1.5% NaHC03 + 63 ppin (1.00)
Table 5: Results of corrosion rate studies for API 5 CT N-80 steel coupons exposed to
Calciuin Bromide and Zinc Bromide brine (specific gravity 1.70) at 1 50°C
S. I Medium I Corrosion Rate I Inhibition
No lnPY (mm/~) 'Yo
1 Calcium Bromide + Zinc Bromide 268.60 0.00
brine (specific gravity 1.70) (6.72)
2 Calcium Bromide + Zinc Bromide 4.75 98.23
brine (specific gravity 1.70) (0.12)
+ 1.5% NaHC03 + 63 ppm of
Sodium sulphite +3%AC1 (vlv)
Table 6: Results of corrosion rate studies for API 5 CT N-80 steel coupons exposed
to Calcium Bromide and Zinc Bromide brine (specific gravity 2.00) at 1 50°C
S. I Medium I Corrosion Rate 1 lnhibition
No mpy (ininfy) YO
1 Calciui~Bl romide + Zinc Bromide 1450.3 0.00
brine (specific gravity 2.00) (36.25)
2 Calcium Bromide + Zinc Bromide 4.35 99.70
brine (specific gravity 2.00) + 1.5% (0.1 1 )
NaHC03 + 63 ppm of Sodium sulphite
+ 3%ACI (vlv)
[0019] Results for corrosion rate studies cnumeratcd in tables 1-6 show significant
decrease in corrosion rate and increase in corrosion inhibition percentage when well
conlpletion fluids prepared using the process of the present invention are used.
5 [0020] While the exelnplary embodiments of the present invention are described and
illustrated herein, it will be appreciated that they are merely illustrative. It will be
understood by tl~oses killed in the art that various moditications in form and detail
may be made therein without departing from or offending the spirit and scope of the
invention as defined by the appended claims.
10
We claim:
1. A process for preparing a corrosion inhibited completion fluid for oil and gas wells,
the process comprising the steps of:
adding a pH n~oderatinga gent to water;
adding an oxygen scavenger to the solution containing water and the pH
moderating agent;
10
blending one or more salts to the solution after the step of adding the oxygen
scavenger, wherein the one or more salts comprise at least one of zinc bromide
and calcium bromide; and
15 adding an acid corrosioll inhibitor after the step of blending the one or more
salts.
2. The process of claim 1, wherein the pH moderating agent is sodium bicarbonate in
an amount of 1.5%.
2 0
3. The process of claim 1, wherein the oxygen scavenger is sodium sulpl~itei n an
amount of 63 milligrams per litre.
4. The process of claim 1, wherein quantity of the one or more salts blended with the
25 solution is based on required specific gravity of the completion fluid.
5. The process of claim 1, wherein specific gravity of the prepared corrosion inhibited
conlpletion fluid is between I .20 to 2.00.
5 6. The process of claim I, wherein 3'X, acid corrosion inhibitor is added after the step
of blending the one or nlore salts.
7. A corrosion inhibited completion fluid comprising:
water as a base fluid;
10 a pH moderating agent;
an oxygen scavenger;
one or more salts, wherein the one or more salts comprise at least one of zinc bromide
and calcium bromide; and
an acid corrosion inhibitor.
15
8. The corrosion inhibited completion fluid of claim 7, wherein the pH lnoderating
agent is sodiuln bicarbonate in an amount of 1.5%.
9. The corrosion inhibited completion fluid of claim 7, wherein the oxygen scavenger
20 is sodiurn sulphite in an amount of 63 lnilligrams per litre.
10. The corrosion inhibited completion fluid of claim 7, wherein quantity of the one
or more salts in the completion fluid is based on required specific gravity of the
completion fluid.
11. The corrosioll inhibited completioi~f luid of claim 7, whcrcin thc specific gravity
of the corrosion inhibited completion fluid is between 1.20 to 2.00.
12. The con-osion inhibited completion fluid of clainl 7, wherein the colnpletion fluid
5 comprise 3% acid corrosion inhibitor.